Add platform identificators for FreeBSD and DragonFly BSD

Hello,

We hold this local patches for libturpial in corresponding FreeBSD port, so I'd like to upstream it and remove it from the port. This change adds platform identificators for FreeBSD and DragonFly BSD. Thanks.

Here is the patches in FreeBSD port for the reference: 
http://svnweb.freebsd.org/ports/head/net-im/py-libturpial/files/patch-libturpial__common____init__.py?revision=345665&view=markup
http://svnweb.freebsd.org/ports/head/net-im/py-libturpial/files/patch-libturpial__common__tools.py?revision=345665&view=markup

I also added test case for that.